Most of the electrocoating paints of high throw and good corrosion protection customary these days consist of amino-epoxy resins based on bisphenol A (2,2-bis(4-hydroxyphenyl)propane) and amines and/or amino alcohols, as described for example in DE-A-3,422,457, DE-A-3,325,061 and DE-A-3,444,110. The crosslinking of these binders is usually thermal in the course of baking at from 120.degree. to 200.degree. C. Crosslinking can be effected by reaction with blocked polyisocyanates as described for example in DE-A-2,057,799, DE-A-2,131,060, DE-A-2,252,536, DE-A-2,265,195, DE-A-2,363,074 and DE-A-2,634,211. Urea condensation products as described in DE-A-3,311,514 can likewise be used.
Another kind of crosslinking consists in the use of phenolic Mannich bases as described for example in DE-A-3,422,257. To obtain sufficiently low baking temperatures if these crosslinking agents are used, binders having a relatively high proportion of primary or secondary amino groups are required.
Although the coating compositions thus obtained have on the whole a positive range of properties, they do show defects in certain areas which appear to disqualify them from use in particularly demanding sectors, for example the coating of car bodies in the automotive industry. Binder systems based on blocked isocyanates or urea condensation products showed appreciable differences in adhesion to differently pretreated steels. On crosslinking with Mannich bases it was in particular the adhesion to nonpretreated metal and the thickness of the paint film obtained which were unsatisfactory.